Plastic (polymer) materials have received considerable attention as a new medium for use in photonic-based electronics for at least the last two decades. Photovoltaic devices such as, for example, solar cells, convert electromagnetic radiation into electricity by producing a photo-generated current when connected across a load and exposed to light. Polymers and their composite derivatives have high commercial potential for use in such photovoltaic devices due to their favorable optical properties. First, certain polymers can convert almost all resonant light into energy through charge carrier generation. Second, the optical absorption of the polymers and their composite derivatives can be tailored to provide a desired bandgap. For instance, a bandgap of 1.1 eV is present in today's silicon-based photovoltaic devices. Third, simple and cost-effective production techniques are well established in the manufacturing arts for making polymer thin films.
Even though the first plastic solar cells were fabricated about twenty years ago, conversion efficiencies for polymer-based photovoltaic devices have yet to match those of inorganic thin film photovoltaic devices. Current polymer-based photovoltaic devices typically display conversion efficiencies of only slightly greater than 5%. In contrast, commercial photovoltaic devices utilizing crystalline or amorphous silicon commonly have conversion efficiencies greater than 20% for crystalline silicon and between 4 to 12% for amorphous silicon.
There are several reasons that polymer-based photovoltaic devices have failed to function at high efficiencies. A first reason is poor charge carrier transport. Although polymers and polymer composites can convert almost all resonant light into charge carriers (electrons, holes or excitons), carrier transport is generally poor. Poor charge carrier transport arises for at least the following two reasons: 1) Excitons travel only very short distances (typically about 50 nm) before being recombined; and 2) Polymer-based photovoltaic materials generally show poor carrier mobilities and conductivities. As a consequence of poor charge carrier transport, polymer-based photovoltaic devices have typically been fabricated from ultra-thin, semi-conductive polymer films, typically less than about 250 nm. As a further consequence of having such ultra-thin, semi-conductive polymer films, significant incident light upon the devices is lost due to transparency. Another substantial drawback of polymer-based photovoltaic devices is the propensity for polymer-based photovoltaic materials to undergo oxidative degradation. Accordingly, stringently-controlled assembly conditions and active device protection are often needed when working with the polymer-based photovoltaic materials. Finally, although the absorption range of polymer-based photovoltaic devices can be adjusted through chemical modification of the polymer or polymer composite, the absorption range of any given organic material is inherently limited to only certain regions of the electromagnetic spectrum. Accordingly, only a portion of the electromagnetic spectrum is capable of interacting with the photovoltaic device containing a given photovoltaic material.
As completely different issues are associated with polymer-based photovoltaic materials compared to inorganic photovoltaic materials, it is apparent from the foregoing that a new approach and architecture for working with polymer-based photovoltaic devices would be of substantial benefit in the art.